Magnetic Resonance Imaging (MRI) is a medical imaging technique commonly used in radiology to visualize internal body structures and functions, offering extraordinary tissue discrimination. Since its inception from the pioneering research efforts of Dr. Raymond Damadian, the advantages of MRI over other imaging techniques, e.g., computed tomography (CT), also known as Computerized Axial Tomography or CAT scanning and based on X-ray radiography, are many. For example, MRI provides much greater tissue contrasts and employs no ionizing radiation. These and numerous other advantages have made MRI the imaging tool of choice in medical diagnoses and treatment regimens.
In magnetic resonance imaging, the body of a subject is positioned in a primary field magnet and subjected to a strong, constant magnetic field. Radio frequency signals are applied to the subject, which causes the axes of certain atomic nuclei within the body of the subject, usually hydrogen atomic nuclei, to precess or rotate around axes parallel to the direction of the magnetic field. The precessing nuclei emit weak radio frequency signals referred to herein as magnetic resonance signals, which are collected and utilized in magnetic resonance imaging.
More particularly, by applying small magnetic field gradients to the subject along with the static magnetic field, particular magnetic resonance signals can be identified and spatially encoded so that it is possible to recover information about individual volume elements or “voxels” within the subject's body from the magnetic resonance signals. This information can be employed to reconstruct an image of internal structures within the body. Because magnetic resonance imaging is non-invasive and does not use ionizing radiation, it is inherently safe. Moreover, magnetic resonance imaging can provide excellent images depicting structures which are difficult to image using other modalities, e.g., the aforementioned X-rays, CAT scans and other imaging techniques.
In his research, Dr. Damadian found that diseased tissues, such as tumors, can be differentiated from normal tissue because the protons, e.g., in hydrogen nuclei, in different tissues return to their rest or equilibrium state at different rates. In other words, the relaxation times of the hydrogen nuclei in normal and diseased tissue differ markedly. Through modifying the parameters in an MRI scan this remarkable effect is employed to distinguish between varying tissue types in a body, providing a non-invasive and harmless window to internal body structures and functions. With more recent advances, functional MRI (fMRI) and other dynamic techniques have opened imaging further.
The quality of a magnetic resonance image strongly depends on the quality and homogeneity of the static magnetic field. To provide an optimum image, the static magnetic field must be both strong (typically on the order of 0.5 Tesla or more) and uniform to about 1 part in a million or better, more particularly to about one part in ten million or better. Some magnetic resonance imaging instruments employ air core superconducting static field magnets. As is understood in the art, these magnets typically have electromagnet coils formed from superconducting materials arrayed along an axis so that the coils cooperatively form an elongated solenoid surrounding the axis. In contrast to other types of MRI scanners, such as the UpRight® scanner made by Fonar Corporation, the coils are wrapped in close proximity to a patient-receiving space to reduce the Ampere turns required for a given field strength. The coils are cooled to cryogenic temperatures, typically about 4.2° Kelvin (approximately −267° C. or −450° F.) or perhaps higher temperatures with newer superconductive materials that can operate at higher temperatures. At cryogenic temperatures, however, the coils effectively have no electrical resistance. Liquid helium is often employed in such cryogenic systems, and the superconducting coils so cooled can conduct large electrical currents and provide a strong magnetic field. In fact, once the coils have been cooled to superconducting temperatures, electrical current flows without resistance and larger currents are possible.
Magnets of this type typically have a housing defining an elongated bore therethrough extending along an axis and require that the patient enter into this bore. The bore may be about 1 meter in diameter or smaller. Thus, the patient is subjected to a highly claustrophobic experience during imaging, akin to some as lying on a stretcher inside a drain pipe. Moreover, these “tube” devices typically cannot be used to image patients who are extremely obese, who require bulky life support equipment during imaging or are claustrophobic. More problematically, air core magnets of this design have strong magnetic fringe fields extending well outside of the magnet housing. These fields can attract ferromagnetic objects in the vicinity of the magnets with such strength that the objects can turn into deadly missiles. Despite stringent precautions taken by imaging centers to prevent entry of ferromagnetic objects into the danger zone surrounding a magnet, accidents have occurred, resulting in injuries and deaths. Some magnet designs wrap a reverse winding outside the primary or exciting winding to suppress fringing fields.
Iron core magnets, as their name suggests, use a ferromagnetic frame that defines a magnetic flux path, and usually include ferromagnetic poles projecting towards a patient-receiving space from opposite sides, such that the pole tips define the patient-receiving space between them. Because of the rather unique physical properties of elemental Iron, the ferromagnetic frame effectively eliminates the fringe field outside of the frame. Moreover, the ferromagnetic frame serves to better concentrate the primary magnetic field within the patient-receiving space and provides a low-reluctance flux path. Ferromagnetic frame magnets can provide the requisite field strength using essentially any source of magnetic flux, including superconducting coils, resistive coils, or masses of permanent magnet material.
One particularly desirable ferromagnetic frame magnet is disclosed in commonly-owned U.S. Pat. No. 6,677,753, the disclosure of which is hereby incorporated by reference herein. As disclosed in preferred embodiments of the '753 patent, the frame includes ferromagnetic side walls extending generally vertically and flux return structures extending generally horizontally above and below the patient-receiving space. Poles project from the side walls toward the patient-receiving space. As described in greater detail in the '753 patent and further hereinbelow, a patient may be positioned within the patient-receiving space in essentially any orientation relative to gravity, and may be moved relative to the frame so as to position essentially any part of the patient's body within the patient-receiving space in the vicinity of the magnet axis extending between the poles.
Preferred magnets according to this general structure can provide extraordinary imaging versatility. For example, a patient may be imaged lying in a recumbent, substantially horizontal position, and then imaged again while in a substantially vertical position, such as standing or sitting. Comparison of these images can yield significant information about certain conditions, e.g., in load-bearing situations. Also, these magnets provide an open environment for the patient, avoiding some of the more claustrophobic aspects. Although the tubular enclosures of prior systems offer simplicity in design, the physiological response of patients and aesthetics demand a better approach, which was championed again by Dr. Raymond Damadian in his UpRight® MRI system design and configuration.
As mentioned above, a ferromagnetic frame magnet can use any source of magnetic flux, including superconducting coils, and a magnet using superconducting coils in conjunction with the ferromagnetic frame can provide very high field strength with good uniformity. For example, U.S. Pat. Nos. 6,323,749 and 4,766,378, incorporated herein by reference, disclose particular arrangements for mounting superconducting coils on ferromagnetic frame magnets.
Despite all of this progress in the magnet art and increasing the field strength, still further improvement would be desirable. For example, as mentioned above, superconducting coils must be maintained at cryogenic temperatures, typically at the temperature of liquid helium (about 4.2° Kelvin or below). Typically, the coils are provided with refrigeration units referred to herein as cryocoolers, which can extract heat directly from the coil and from associated components even at this very low temperature, constituting immersion cooling. However, the cryocoolers have only a very limited capacity, typically on the order of a few watts or less at this temperature. Therefore, the coils themselves must be surrounded by very efficient thermal insulation material to remain effective. Most commonly, the coils are enclosed in vessels which are maintained under hard vacuum, by which the coolant can contact the coils. The coils must be supported and rigidly held in place within the vessels. The structures holding the coils must resist not only the weight of the coils but also the enormous magnetic forces generated during operations. Depending on the particular coil design and the design of any adjacent ferromagnetic frame elements, these forces can be on the order of tons. The coil vessel and supporting structure should also be compact so as to minimize the size and weight of the apparatus. Designing a compact coil enclosure and supporting system has presented a challenge to those of skill in the art.
Additionally, Applicants note that cryocoolers in operation, primarily due to their direct contact with the cooled component, typically induce mechanical vibrations. Transmission of such vibrations to the coils and associated structures tends to undermine the careful calibration of the coils and degrade the uniformity and stability of the static magnetic field. However, it is generally desirable to position portions of the cryocooler in proximity to the coil enclosure to better extract heat and maintain the superconductivity of the system. This, however, further complicates design of such a system. Additionally, although the benefits are substantial, the superconducting coils used heretofore typically have not been arranged for optimum co-action with the ferromagnetic frame. Thus, further improvements to combine and enhance such combinations would also be desirable.
Furthermore, Iron-based systems have functional limits in field strength that at present cannot be overcome, and as higher field strength systems become available with the decreasing cost and increasing temperature of superconducting materials, the inherent limitations of Iron-based systems prevents or hinders utilization of the newest technologies and imaging techniques employing greater and greater field strengths.
Whether having an air core or ferromagnetic frame, MRI devices that improve the image quality are desirable. By minimizing outlier fringe field lines and strenghthening the uniformity of the constant primary field lines, images can be improved, along with diagnoses.
There is, therefore, a need for reduced outlier fringe fields and improved uniformity of magnetic field lines at higher field strengths, whether employing air core, ferromagnetic or combinations thereof.
The present invention offers solutions to overcoming these problems and options to utilize the best of both paradigms.